Schistosomiasis is a tropical parasitic disease caused by infections with flukes of the genus Schistosoma, affecting 200 million individuals worldwide. A new drug for schistosomiasis is urgently needed as praziquantel is currently the drug of last resort and the development of resistance cannot be ignored, particularly in view of its large-scale use in many endemic countries. Our long-term goal is to discover a new orally active single- dose antischistosomal drug with activities against all parasite stages and with a novel mechanism of action. The objective of this proposal, the first step in pursuit of this goal, is to se the orally active aryl hydantoin prototype Ro 13-3978, a close structural analogue of the androgen receptor (AR) antagonist nilutamide, as a starting point to identify one or more lead compounds with high antischistosomal efficacy and with minimal to no interaction with the host AR. Our central hypothesis is that for aryl hydantoins and related heterocycles, the structural requirements for antischistosomal efficacy and AR binding interactions are divergent. This hypothesis arose on the basis of preliminary data produced in the applicant's laboratories. The rationale that underlies this research is to separate the desirable multi-stage antischistosomal properties of aryl hydantoins and related heterocycles from the undesirable antiandrogenic side effects in the host. Our central hypothesis will be tested by pursuing three specific aims: 1) To synthesize and characterize a structurally diverse library of Ro 13-3978 analogs; 2) To assess schistosomicidal activities of target compounds against S. mansoni; and 3) To determine target compound AR binding affinity [and cytotoxicity.] Our target compound design maximizes structural diversity guided by incorporation of substructures and functional groups known to diminish ligand-AR interactions. This approach is innovative because our strategy to decrease side effects caused by host AR antagonism capitalizes on negative SAR data gleaned from AR ligand binding studies to decrease, not increase, AR binding affinity. The expected outcomes from this work are as follows. First, we will gain a new understanding of the structural specificit of AR binding vs. antischistosomal efficacy for aryl hydantoins and related heterocycles. Second, one or more structurally novel orally-active lead compounds with high antischistosomal selectivity will likely be identified. This proposed research is significant because it will provid the required data to justify the more labor-intensive multi-dimensional lead optimization effort to discover a new and inexpensive orally active single-dose antischistosomal drug with activities against all parasite stages and with a novel mechanism of action. Such a drug would be important in the chemotherapy of drug-resistant schistosomiasis and likely be valuable in integrated control programs to curb schistosomiasis.